The invention relates to an apparatus for sealing shut-off devices located in pipelines for transporting liquids and gases, and in particular, a plug for a ball valve.
Ball valves used in pipeline construction have a casing insertable into the pipeline and a spherical plug rotatable about an axis in the casing forming the shut-off device. The plug is sealable against the casing by seals pressing against the circumferential surface of the plug.
Sealing ring systems mounted in a floating manner in which a gasket, for example of Teflon, is clipped in a seating ring and projects, and O-rings or the like sealing the seating ring system with respect to the casing are known. Spring elements are positioned between the seating ring and the casing, and by a permanent pressing action, ensures the necessary surface pressure between the plug and the gasket. The seating ring surfaces exposed to the pipeline pressure are so dimensioned that as a result of the pipeline pressure acting on the surfaces, there is a pressure-dependent sealing force which, together with the spring tension, forms the overall sealing force acting on the gasket. These known seating rings act in a self-relieving manner (double block and bleed), that upon exceeding the permitted casing internal pressure, the seating rings are briefly raised from the spherical plug so that there is a pressure compensation to the pipeline.
There are known problems with double-acting seating ring systems (double piston) because the seating rings also seal on the outflow side, so that without additional units, a self-relief is impossible.
These known systems suffer from the disadvantage that the seating ring is not raised or relieved prior to the switching or operation of the spherical plug. This causes the thrust collar to scrape on the circumferential surface increasing the wear and thus reducing the life of both the plug and the seating ring system. In addition, the seating ring system can only be pressed onto the spherical plug as a function of the predetermined differential pressure surfaces and the spring tension. DE 29 38 265 A1 discloses a regulatable system for the contact pressure of seals in ball valves in which the seating ring carrying the gasket or packing is constructed as a piston with a central flange. Two chambers are formed between the casing and the flange surface into which a pressure medium can be introduced. As a function of the pressure acting on the particular flange surface, the seating ring is pressed against the circumferential surface of the spherical plug or is raised therefrom. However, such a system no longer functions if the pressure medium regulation or control system fails, for example in the case of a fire.